During construction it is often desirable to drill horizontal holes in rock under roadways and building foundations without disturbing those structures. Such horizontal holes may be used for utility services, or drainage, or the like, and for this purpose it is desirable to have a straight hole economically drilled with a minimum of ground disturbance.
To accomplish such holes it is common to dig a shaft on either side of the structure and by means of auger in soft ground, or a rock drill in harder ground, drive the hole from shaft to shaft. This is accomplished by utilizing conventional drilling techniques which push the auger or rock drill utilizing a rotary drill string. The auger or drill and trailing drill string is inserted in sections and driven from one of the shafts to the other.
Core drills have been developed for small diameter holes which are relatively effective in drilling accurate straight holes between two locations. However, effective core drilling is limited to holes of about 4 to 8 inches or smaller because the cores must be drilled in segments, usually a maximum of twenty (20) ft. and removed.
In larger size holes the process would be slow and the cores become increasingly difficult to remove, particularly in hard rock. As a result, a common practice has developed in boring larger diameter holes to first bore a cored hole and thereafter force a series of rotary ring reamers through the hole to increase the hole size to the diameter desired.
Reaming is also time-consuming and in general produces a hole which is rough and/or cork screwed and as the size increases it becomes more difficult to maintain the progress without buckling the drill string creating the rotary drive and force on the ring cutters. An improvement to this method involves the pulling of the ring cutters back through the hole but since this is a progressive stepwise operation the difficulty of the reamer walking about the hole diameter, although improved, is still limited as set forth above. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the foregoing limitations.